Declining groundwater resources in semi-arid areas are often cited because of anthropization and climate change. This is not the case in Nouakchott (Mauritania) where the water level has risen by 1 to 2 m over the last 40 years in parallel with urban expansion (+1 million inhabitants in 60 years). Using former and new data, primarily water table measurements and chemical indicators (major ions, bromide, 18 O, 2 H), we show that the groundwater level rise is mainly a consequence of the rapid population growth in the Nouakchott area, while the global sea level rise only has a limited impact. The increased supply of domestic water (currently 120,000 m 3 /day) and the lack of waste water networks have added large amounts of water to the Quaternary aquifer. In this metropolis where 60% of the total area is at an elevation of less than 1 m asl, the rise in the groundwater level has dramatic consequences, including the abandonment of flooded districts, and the emergence of new diseases.

Means of groundwater recharge in a semi-arid area: example of the Trarza aquifer (southwest Mauritania) Abstract The Trarza aquifer extends over about 40 000 km 2 in southwestern Mauritania, between the Senegal River in the south, the Atlantic Ocean in the west and the Mauritanides metamorphic chain in the north and east. The Trarza aquifer lies in Continental Terminal and Quaternary sediments. The field investigations conducted between 2010 and 2012 significantly added to the few older measurements. The hydrodynamic and geochemical approaches showed that in this semi-arid area, the groundwater dynamics is influenced by many environmental changes, past and present (from Quaternary transgressions to the various dams regulating the Senegal River). Groundwater is mainly recharged by lateral infiltration of surface water of the Senegal River, and to a lesser extent, by rainfall infiltration. Groundwater mineralization is controlled by water-rock interactions with silicates and alumino-silicate minerals and is locally influenced by marine water resulting from past Quaternary transgressions. Evaporation strongly affects surface waters before and during their infiltration. Very preliminary calculations suggest an annual recharge of less than or equal to 5% of precipitation, i.e. about 5 to 10 mm.